**4. Factors affecting the anaerobic digestion process stability**

The factors affecting the production of biogas are mainly based on the operating conditions of the digester, such as pH and temperature which influence directly the micro-organisms. The perturbations in effluent (including the concentration of substrate and its composition in toxic compounds and inhibitors) can also affect the volume and the quality of the produced biogas. Sometimes, the toxic compounds are not present at the beginning in the effluent waste, but they are produced inside the digester starting from degradation of substrate (example: VFA and ammonia).
